It is well known that organopolysiloxanes heated in the presence of an organic peroxide are converted to silicone rubbers excellent in heat resistance, cold resistance, weatherability and electric characteristics. It is also known that organopolysiloxanes are cured by light irradiation in the presence of a photoinitiator.
In JP-B S52-40334 (Patent Document 1) and JP-A S60-104158 (Patent Document 2), for example, there are disclosed UV-curing organopolysiloxane compositions which contain a vinyl group-containing polysiloxane and a mercapto group-containing polysiloxane and are able to form a cured product through photoradical addition reaction. In JP-B S53-36515 (Patent Document 3) and JP-A S60-215009 (Patent Document 4), there are also disclosed photocurable organopolysiloxane compositions which contain an acrylic group-containing polysiloxane and a sensitizer and are able to form a cured product by photoirradiation.
In JP 2639286 (Patent Document 5), there is disclosed a photocurable organopolysiloxane composition making use of a curable organopolysiloxane compound obtained by reaction between a hydroxy-terminated organopolysiloxane and a (meth)acrylic functional alkoxysilane.
Further, in JP 2782405 (Patent Document 6), there is disclosed a photocurable organopolysiloxane composition making use of a curable organopolysiloxane compound obtained by reaction of an organopolysiloxane having a terminal hydrolyzable silyl group and an acryloxyalkylsilanol.
However, the above compositions containing both a vinyl group-containing polysiloxane and a mercapto group-containing polysiloxane have a problem on the odor and metal corrosiveness of the mercapto group, with the attendant drawback that their use is limitative.
With respect to the composition comprising an acrylic group-containing polysiloxane and a sensitizer, it is needed to use a linear polysiloxane having a high molecular weight as an acrylic group-containing polysiloxane so as to obtain a rubbery elastomer. Hence, the amount of the acrylic group positioned at terminal ends in the polysiloxane becomes relatively very small, so that curability lowers. Another disadvantage resides in that the surface portion exposed to air scarcely undergoes curing owing to the curing hindrance of oxygen. Accordingly, this sort of composition has been put into practice only for obtaining a resinous cured product by use of a polysiloxane having a relatively large amount of acrylic group, and has never been put into practical use for obtaining a rubbery elastomer at present.
Further, where a curable organopolysiloxane compound obtained by reaction between the hydroxy-terminated organopolysiloxane and a (meth)acrylic functional alkoxysilane is formed into a cured product by photoirradiation, the curability of a polymer modified with one (meth)acrylic group at each end is insufficient along with a disadvantage in that depth curability is poor.
Still further, where there is used an organopolysiloxane compound obtained by reaction between an organopolysiloxane having a terminal hydrolyzable silyl group and an acryloxyalkyl silanol, curability with photoirradiation is good. However, the acryloxyalkylsilanol that is a starting material for obtaining the curable organopolysiloxane compound is instable, thus presenting problems on the feed stability of the starting material, preparation stability and storage stability of the resulting composition.